1. Field of the Invention
This invention relates to an improved process for separating acetic acid and methyl acetate from water using certain organic compounds as the azeotropic agent in azeotropic distillation. Specifically, in a process for manufacturing an aromatic carboxylic acid by oxidizing an aromatic alkyl hydrocarbon, this invention is concerned with a method of azeotropic distillation using the said aromatic alkyl hydrocarbon as the azeotropic agent for the separation of acetic acid and methyl acetate from water.
2. Description of the Related Art
Water and methyl acetate are produced as by-products in the process of manufacturing an aromatic carboxylic acid, such as terephthalic acid and isophthalic acid, wherein an aromatic alkyl hydrocarbon such as p-xylene and m-xylene, respectively, is oxidized with air in the presence of an acetic acid solvent and a catalyst system comprised of cobalt, manganese and bromine. A portion of the condensed liquid from the vapor stream of the oxidation reactor and other dilute acetic acid streams, containing mostly water and methyl acetate, are sent to the dehydration unit to recover concentrated acetic acid and to remove the oxidation by-products, which are mainly water and methyl acetate.
In the above-described method of the prior art for the commercial production of aromatic carboxylic acids, the conventional distillation method is widely used in the dehydration unit to recover concentrated acetic acid and to remove water and methyl acetate from the system. The recovered acetic acid is recycled back to the oxidation reactor for reuse as the solvent, while water and methyl acetate are sent to the wastewater treatment system. This method has several drawbacks such as low recovery rate of acetic acid, excessive steam consumption, total loss of methyl acetate, and very high wastewater treatment costs due to the high concentration of acetic acid and methyl acetate in wastewater. Furthermore, unreacted aromatic alkyl hydrocarbons present in the feed stream to the dehydration unit are lost to wastewater, resulting in a lower yield of aromatic carboxylic acid.
U.S. Pat. No. 4,250,330 describes an azeotropic distillation process for the dehydration unit to separate acetic acid and methyl acetate from water, in which isobutyl acetate is used as the azeotropic agent. In this process, methyl acetate is recovered from the top of the stripping column and then the recovered methyl acetate is recycled to the oxidation reactor of p-xylene. Despite some improvements over the conventional distillation method such as lower steam consumption and recovery of methyl acetate, this process has the following drawbacks:                (a) A small amount of isobutyl acetate contained in the streams of recovered acetic acid and methyl acetate is recycled back to the p-xylene oxidation reactor and is then decomposed to isobutanol, propanol, methanol, and carbon oxides (CO and CO2). Therefore, the consumption rate of isobutyl acetate is high. The decomposition products, isobutanol, propanol and methanol, further react with organic radicals in the oxidation reactor, producing a number of impurities, which contaminate the process streams and the terephthalic acid product. Unreacted isobutanol, propanol and methanol are then introduced into the dehydration unit along with the dilute acetic acid streams. These alcohol compounds significantly affect the performance of the dehydration unit, causing poor separation and higher consumption of steam and cooling water.        (b) Small amounts of p-xylene and isobutanol contained in the feed to the dehydration unit build up to high concentrations in the organic phase because removal of these chemicals from the dehydration unit is very difficult. Large amounts of p-xylene and isobutanol circulating in the dehydration unit increase consumption of steam and cooling water and lower the separation efficiency.        
U.S. Pat. No. 5,980,696 describes a method of using isobutyl acetate or n-propyl acetate as the azeotropic agent to separate water from acetic acid by azeotropic distillation, but this process also has similar drawbacks to those mentioned above for U.S. Pat. No. 4,250,330.